In order to reduce further the fuel consumption and the emissions of pollutants of modern internal combustion engines, internal combustion engines are being increasingly used which have a variable valve lift system for variable control of the lift of the inlet valves and/or of the outlet valves. In these systems, load control without throttling by means of a throttle valve is possible over a large operating range of the internal combustion engine since the load control takes place by means of the lift of the inlet valves. In particular, internal combustion engines with a variable valve lift system can be operated virtually unthrottled in a lower and central load range, i.e. the throttle valve is fully opened. In this unthrottled operating range, for example losses due to load changes are low, which can bring about reduced consumption of the internal combustion engine.
Fabrication-related deviations and tolerances in a valve train and in the valves can lead to errors during the filling of the cylinders. In particular, in the lower load range and in the idling mode, where the lifts of the inlet valves are very small in the unthrottled operating mode, even small errors during the filling of the cylinders can result as perceptible differences in torque which limits driving comfort. These differences in torque, which can occur not only between different internal combustion engines but also between the cylinders of the same internal combustion engine, lead in the worst case to perceptible unsmooth running in the lower load range or in the idling mode and therefore limit the driving comfort. In addition, errors during the filling of a cylinder, owing to tolerances in the valve lift, can affect the lambda values of the individual cylinders of the internal combustion engine.
EP 1 193 381 A2 discloses a control unit for controlling an internal combustion engine with a variably controllable valve lift and with a throttle valve in the intake section. The control unit has means which monitor the unsmooth running of the internal combustion engine and which, in the event of a defined unsmooth running threshold value being exceeded and in the case of a valve lift which is smaller than a predefined threshold, activate the throttle valve in the closing direction until the unsmooth running threshold value is undershot again.
In addition, various methods for compensating cylinder-specific differences in the lambda value are known from the prior art. For example, DE 10 2005 009 101 B3 describes a method for determining a correction value for influencing an air/fuel ratio in a respective cylinder of an internal combustion engine with a plurality of cylinders. The cylinders are assigned injection valves which meter fuel. An exhaust gas probe supplies a measurement signal which is characteristic of the air/fuel ratio in the respective cylinder. The measurement signal is detected at a predefined sampling crankshaft angle with respect to a reference position of the piston of the respective cylinder and is assigned to the respective cylinder. By means of one controller in each case, a controller value for influencing the air/fuel ratio in the respective cylinder is obtained as a function of the measurement signal which is detected for the respective cylinder.
Document DE 10 2006 026 390 A1 describes an electronic control device for controlling an internal combustion engine in a motor vehicle with an unsmooth running determining unit and with an injection quantity correction unit, wherein a defined group of cylinders is assigned to a lambda probe. The control device is characterized in that the injection quantity correction unit is configured in such a way that the injection quantity of a cylinder, which is to be examined, of the defined group can be adjusted in the lean direction by a difference adjustment value which is assigned to an unsmooth running difference value, and the injection quantity of at least one of the other cylinders which are assigned to the same lambda probe can be correspondingly adjusted in the enriched direction, with the result that overall a predefined lambda value of this group, preferably a lambda value of at least approximately 1, is achieved. In addition, the control device is characterized in that in this way a cylinder-specific difference adjustment value can be set for each cylinder of the defined group, and in that cylinder-specific correction values can be determined by forming a ratio between the cylinder-specific difference adjustment values.
WO 2006/018377 discloses a method for controlling an internal combustion engine. The internal combustion engine has a camshaft which acts on gas exchange valves, a phase adjustment device by means of which a phase between the camshaft and a crankshaft can be adjusted, an exhaust gas probe by means of which a variable which characterizes an air/fuel ratio in a cylinder is detected, at least one sensor for detecting the phase and at least one actuating element which acts on the internal combustion engine. In the method, measurement data sets are determined which are assigned to various detected phases and which comprise, in addition to the detected phase, at least the detected variable which characterizes the air/fuel ratio in the cylinder. In addition, an optimization method is carried out by means of which a correction value for the detected phase is obtained as a function of the measurement data sets, specifically in such a way that a quality function is minimized or maximized, said quality function depending on the variables assigned to the measurement data sets. In addition, in the method at least one actuation variable for controlling an actuating element is determined during the further operation of the internal combustion engine, as a function of the detected phase which is corrected by means of the correction value.